People notice the best looking lawn on the block in a second. Make the lawns you work on THAT LAWN! Then people will start to ask, who's lawn is that, who works on it? Then you advertise your name, and get it out there. People should see your name everywhere, until it is common to associate the nice lawns with your business.
I have been using door hangers in the specific areas in which I want to work. Then as I get one or two customers from that, I try to network the area a little more, to keep drive time down.
This year, I added putting business cards all over town at local small businesses. They seem to not miind, because they also are small business owners. Another thing I am doing is teaming up with a local pizza guy, and he has 5000 door hangers going out, and I piggy backed some flyers and coupons along with his ads, and we are hitting a general area that way. Only paying about $0.02 per flyer, so that's much cheaper than I can do myself.
Also, the lawn signs are a great idea. I have not tried them yet, but am going to look into offering like a free mowing to put them in the customers yard, or simply ask them if they'd mind. i think that is a good way to get people's attention.